1. Field of the Invention
Aspects of the present invention relate to a liquid tank within a fuel cell system, and more particularly, to a liquid tank capable of detecting a liquid level even when the fuel cell system is rotated.
2. Description of the Related Art
In general, a fuel cell is a power generation system that directly converts chemical energy into electrical energy by the electrochemical reaction of hydrogen and oxygen. Hydrogen is supplied to the fuel cell by reforming methanol, ethanol, and natural gas, etc. Oxygen is generally supplied thereto from air by using an air pump, etc.
Fuel cells include polymer electrolyte fuel cells and direct methanol fuel cells operated at a temperature of about 100° C. or less, phosphoric acid fuel cells operated at about 150° C. to 200° C., molten carbonate fuel cells operated at high temperatures of about 600° C. to 700° C., and solid oxide fuel cells operated at higher temperatures of about 1000° C. or more, etc. These fuel cells generally operate to generate electricity in the same manner but are different in view of the types of fuels, catalysts, and electrolytes used, etc.
Among others, the direct methanol fuel cell (DMFC) uses a fuel that is a mixture of high concentration liquid methanol and water instead of using hydrogen as the fuel. The DMFC is lower in output density than fuel cells using hydrogen directly. However, the DMFC has a high energy density per volume of methanol used, and methanol fuel is easily stored. Also, the DMFC is well adapted to operate at a low output for a long time. Also, the DMFC can be compactly constructed because the DMFC needs no additional devices, such as a reformer to reform fuel to generate hydrogen, etc.
The DMFC includes a membrane electrode assembly (MEA) configured of a polymer electrolyte membrane to which an anode and a cathode are closely adhered to opposite sides. As a polymer electrode membrane, generally fluoropolymers are used. However, the methanol is rapidly soaked into the fluoropolymer membrane when the DMFC uses a high concentration of methanol as fuel causing a crossover phenomenon of transmitting non-reactive methanol to the polymer electrolyte membrane. Accordingly, in order to lower the concentration of methanol, the fuel mixture of methanol and water is supplied to the fuel cell system.
Meanwhile, the polymer electrolyte membrane fuel cell (PEMFC) uses hydrogen generated by reforming substances, such as methanol, ethanol, natural gas, etc., and has a remarkably high output, a low operating temperature, and rapid starting and response characteristics. Therefore, the PEMFC is widely applicable to transportable power sources, such as automobiles, as well as distributed power sources, such as houses or public buildings, etc., and as power sources for small mobile equipment, such as personal digital assistants (PDAs).
The PEMFC requires pure hydrogen to generate electricity. PEMFCs achieve this by using catalyst reactions, such as steam reforming (SR) and water gas shift (WGS), etc. PEMFCs also require the removal of carbon monoxide, which is a byproduct of the above hydrogen-generating methods and which poisons the catalyst of the fuel cell when included in the hydrogen.
For continuous operation of the fuel cell, the fuel mixture for both DMFCs and PEMFCs is continuously supplied. Thus, it is necessary to maintain the fuel mixture at a constant level. Generally, a conventional fuel cell includes a device to maintain the amount of the fuel in a tank at a predetermined liquid level that detects the amount of the fuel using, for example, ultrasonic waves, etc. However, the construction of such fuel cells is complicated, and the liquid level cannot be measured when the fuel cell is rotated.